This invention relates to a process for combustion of coal and other organic fuels at a high temperature and in a mixture of carbon dioxide and oxygen to provide a combustion gas containing carbon dioxide as a major component. More particularly, the invention relates to a process involving the combustion of an organic fuel and a recycle of a portion of the carbon dioxide wherein a reduced quantity of combustion gas is available as a saleable product and is disposed in an environmentally satisfactory way, thereby reducing the so called "greenhouse effect".
In conventional combustion of coal, air serves as the source of oxygen to support combustion and provide a combustion gas containing nitrogen, carbon dioxide, water and other gases released to the atmosphere. Depending on its concentration and ease of separation, the carbon dioxide may have value for use in oil recovery and other purposes.
In the process, nitrogen oxides may be generated from the nitrogen gas in the air and from nitrogen compounds in the fuel. In addition, sulfur oxides from sulfur in the coal are also usually formed in the combustion process. Unless removed, these components are part of the product stream and may have detrimental effects on the value of the product stream. Scrubbers and other techniques for removing the sulfur and nitrogen oxides may be used but in general have disadvantages due to their cost, complexity and reduced efficiency of the entire process.
Efforts have been directed to reduce the content of these oxides in the combustion gas. In one process described in U.S. Pat. No. 4,547,351, the sulfur oxides are converted to alkali metal sulfates in a gas-gas reaction. The resulting sulfates are then cooled to form solid particles prior to passage through heat exchangers to reduce clogging of the heat exchangers. While this process has advantages, it is not specifically designed to remove nitrogen oxides and utilizes air for combustion purposes.
Accordingly, one object of the invention is a reduction in the formation and content of detrimental oxides in combustion gas from an organic fuel. A second object of the invention is a system to facilitate the separation of a carbon dioxide stream from combustion gas. Another object of the invention is the reduction in combustion gas released to the atmosphere. These and other objects will become apparent from the following description.